Innumerable designs of transmissions for cars and light commercial vehicles have been proposed over the years that utilize hydraulic transmissions for multiplying engine torque to accelerate a vehicle from rest and then restore or preferably increase output speed potential. However, such design efforts have typically resulted in hydraulic transmissions that, although of acceptable torque capacity, are undesirably large and heavy. Moreover, such transmissions have exhibited less than optimum efficiency, and thus fuel economy and/or performance suffer.
As an alternative to the purely hydraulic transmissions referred to above, hydromechanical transmissions have been utilized in drivetrains for large off-road construction and military vehicles. Such transmissions are typically of a split power input type, wherein a hydrostatic power unit and a mechanical power unit are driven in parallel by the vehicle engine. The hydrostatic power unit converts its split mechanical input power from the engine into hydrostatic output power that can be infinitely varied in speed and torque over a particular hydrostatic stroke range. This hydrostatic output power is combined in the mechanical power unit with its split mechanical input power from the engine to produce hydromechanical output power in multiple power ranges. The speed and torque in each of the power ranges, initially set by gear ratios of the mechanical power unit, can be infinitely varied by varying the stroke of the hydrostatic power unit.
A properly designed hydromechanical transmission can advantageously provide synchronous range shifting that affords smooth and uninterrupted power flow from engine to driving wheel(s), as the vehicle is accelerated from rest to maximum speed. An additional benefit is that the engine may be operated at or near its peak efficiency output speed, regardless of transmission output speed. However, to achieve the performance required of automobiles in terms of acceleration, speed and fuel economy, the mechanical power unit has traditionally required far too many geartrains, such as spur and pinion gear sets and planetary gear sets. Such mechanical transmissions suffer the same drawbacks as their counterpart hydraulic transmissions in terms of size, weight, efficiency, cost, etc.